The relationship between biological senescence and cell surface function will be investigated in human diploid fibroblasts. Detailed analysis of amino acid transport systems will be performed with early and late passage diploid fibroblasts. These transport studies will include investigations of the kinetics of influx and efflux, patterns of competitive inhibition, inhibition by metabolic poisons, ion requirements, pH dependence, temperature dependence, and the effects of such factors as cell density, hormones and various surface modifying agents. In addition, surface structures of early and late passage human cells will be analyzed by gel chromatography of surface glycopeptides and by other methods.